The ability of NICMOS to perform high accuracy polarimetry is
currently hampered by an uncalibrated residual instrumental polarization at a
level of 1.2−1.5%. To better quantify and characterize this residual we obtained
observations of three polarimetric standard stars at three separate space-craft
roll angles. Combined with archival data, these observations were used to characterize
the residual instrumental polarization to enable NICMOS to reach its
full polarimetric potential. Using these data, we calculate values of the parallel
transmission coefficients that reproduce the ground-based results for the polarimetric
standards. The uncertainties associated with the parallel transmission
coefficients, a result of the photometric repeatability of the observations, dominate
the accuracy of p and θ. However, the new coefficients now enable imaging
polarimetry of targets with p " 1.0% at an accuracy of ±0.6% and ±15°.